Rechargeable lithium-ion (Li-ion) and lithium metal batteries (including Li-sulfur and Li metal-air batteries) are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound as an anode active material (except Li4.4Si, which has a specific capacity of 4,200 mAh/g). Hence, in general, Li metal batteries have a significantly higher energy density than lithium ion batteries.
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having relatively high specific capacities, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode through the electrolyte to the cathode, and the cathode became lithiated. Unfortunately, upon repeated charges/discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately grew to penetrate through the separator, causing internal shorting and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's, giving ways to lithium-ion batteries.
In lithium-ion batteries, pure lithium metal sheet or film was replaced by carbonaceous materials as the anode. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1.
Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost and performance targets. Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li+ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of lithium transition metal oxide- or phosphate-based cathode active material is typically in the range of 140-170 mAh/g. As a result, the specific energy of commercially available Li-ion cells is typically in the range of 120-220 Wh/kg, most typically 150-180 Wh/kg. These specific energy values are two to three times lower than what would be required for battery-powered electric vehicles to be widely accepted.
With the rapid development of hybrid (HEV), plug-in hybrid electric vehicles (HEV), and all-battery electric vehicles (EV), there is an urgent need for anode and cathode materials that provide a rechargeable battery with a significantly higher specific energy, higher energy density, higher rate capability, long cycle life, and safety. One of the most promising energy storage devices is the lithium-sulfur (Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cell consists of elemental sulfur as the positive electrode and lithium as the negative electrode. The lithium-sulfur cell operates with a redox couple, described by the reaction S8+16 Li 8Li2S that lies near 2.2 V with respect to Li+/Li0. This electrochemical potential is approximately ⅔ of that exhibited by conventional positive electrodes (e.g. LiMnO4). However, this shortcoming is more than offset by the very high theoretical capacities of both Li and S. Thus, compared with conventional intercalation-based Li-ion batteries, Li—S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Assuming complete reaction to Li2S, the values of energy densities can approach 2,500 Wh/kg and 2,800 Wh/l, respectively, based on the combined Li and S weight or volume. If based on the total cell weight or volume, the energy densities can reach approximately 1,000 Wh/kg and 1,100 Wh/l, respectively. However, the current Li-sulfur cells reported by industry leaders in sulfur cathode technology have a maximum cell specific energy of 250-400 Wh/kg (based on the total cell weight), which is far below what is possible.
In summary, despite its considerable advantages, the Li—S cell is plagued with several major technical problems that have thus far hindered its widespread commercialization:    (1) Conventional lithium metal cells still have dendrite formation and related internal shorting issues.    (2) Sulfur or sulfur-containing organic compounds are highly insulating, both electrically and ionically. To enable a reversible electrochemical reaction at high current densities or charge/discharge rates, the sulfur must maintain intimate contact with an electrically conductive additive. Various carbon-sulfur composites have been utilized for this purpose, but only with limited success owing to the limited scale of the contact area. Typical reported capacities are between 300 and 550 mAh/g (based on the cathode carbon-sulfur composite weight) at moderate rates.    (3) The cell tends to exhibit significant capacity decay during discharge-charge cycling. This is mainly due to the high solubility of the lithium polysulfide anions formed as reaction intermediates during both discharge and charge processes in the polar organic solvents used in electrolytes. During cycling, the lithium polysulfide anions can migrate through the separator to the Li negative electrode whereupon they are reduced to solid precipitates (Li2S2 and/or Li2S), causing active mass loss. In addition, the solid product that precipitates on the surface of the positive electrode during discharge becomes electrochemically irreversible, which also contributes to active mass loss.    (4) More generally speaking, a significant drawback with cells containing cathodes comprising elemental sulfur, organosulfur and carbon-sulfur materials relates to the dissolution and excessive out-diffusion of soluble sulfides, including polysulfides, organo-sulfides, carbon-sulfides, and carbon-polysulfides (hereinafter referred to as anionic reduction products) from the cathode into the rest of the cell. This phenomenon is commonly referred to as the Shuttle Effect. This process leads to several problems: high self-discharge rates, loss of cathode capacity, corrosion of current collectors and electrical leads leading to loss of electrical contact to active cell components, fouling of the anode surface giving rise to malfunction of the anode, and clogging of the pores in the cell membrane separator which leads to loss of ion transport and large increases in internal resistance in the cell.
In response to these challenges, new electrolytes, protective films for the lithium anode, and solid electrolytes have been developed. Some interesting cathode developments have been reported recently to contain lithium polysulfides; but, their performance still falls short of what is required for practical applications.
For instance, Ji, et al reported that cathodes based on nanostructured sulfur/meso-porous carbon materials could overcome these challenges to a large degree, and exhibit stable, high, reversible capacities with good rate properties and cycling efficiency [Xiulei Ji, Kyu Tae Lee, & Linda F. Nazar, “A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries,” Nature Materials 8, 500-506 (2009)]. However, the fabrication of the proposed highly ordered meso-porous carbon structure requires a tedious and expensive template-assisted process. It is also challenging to load a large proportion of sulfur into these meso-scaled pores using a physical vapor deposition or solution precipitation process.
Zhang, et al. (US Pub. No. 2014/0234702; Aug. 21, 2014) makes use of a chemical reaction method of depositing S particles on surfaces of isolated graphene oxide (GO) sheets. But, this method is incapable of creating a large proportion of S particles on GO surfaces (i.e. typically <66% of S in the GO-S nanocomposite composition). The resulting Li—S cells also exhibit poor rate capability; e.g. the specific capacity of 1,100 mAh/g (based on S weight) at 0.02 C rate is reduced to <450 mAh/g at 1.0 C rate. It may be noted that the highest achievable specific capacity of 1,100 mAh/g represents a sulfur utilization efficiency of only 1,100/1,675=65.7% even at such a low charge/discharge rate (0.02 C means completing the charge or discharge process in 1/0.02=50 hours; 1 C=1 hour, 2 C=½ hours, and 3 C=⅓ hours, etc.) Further, such a S-GO nanocomposite cathode-based Li—S cell exhibits very poor cycle life, with the capacity typically dropping to less than 60% of its original capacity in less than 40 charge/discharge cycles. Such a short cycle life makes this Li—S cell not useful for any practical application. Another chemical reaction method of depositing S particles on graphene oxide surfaces is disclosed by Wang, et al. (US Pub. No. 2013/0171339; Jul. 4, 2013). This Li—S cell still suffers from the same problems.
A solution precipitation method was disclosed by Liu, et al. (US Pub. No. 2012/0088154; Apr. 12, 2012) to prepare graphene-sulfur nanocomposites (having sulfur particles adsorbed on GO surfaces) for use as the cathode material in a Li—S cell. The method entails mixing GO sheets and S in a solvent (CS2) to form a suspension. The solvent is then evaporated to yield a solid nanocomposite, which is then ground to yield nanocomposite powder having primary sulfur particles with an average diameter less than approximately 50 nm. Unfortunately, this method does not appear to be capable of producing S particles less than 40 nm. The resulting Li—S cell exhibits very poor cycle life (a 50% decay in capacity after only 50 cycles). Even when these nanocomposite particles are encapsulated in a polymer, the Li—S cell retains less than 80% of its original capacity after 100 cycles. The cell also exhibits a poor rate capability (specific capacity of 1,050 mAh/g(S wt.) at 0.1 C rate, dropped to <580 mAh/g at 1.0 C rate). Again, this implies that a large proportion of S did not contribute to the lithium storage, resulting in a low S utilization efficiency.
Furthermore, all of the aforementioned methods involve depositing S particles onto surfaces of isolated graphene sheets. The presence of S particles or coating (one of the most insulating materials) adhered to graphene surfaces would make the resulting electrode structure non-conducting when multiple S-bonded graphene sheets are packed together. These S particles prevent graphene sheets from contacting each other, making it impossible for otherwise conducting graphene sheets to form a 3-D network of electron-conducting paths in the cathode. This unintended and unexpected outcome is another reason why these prior art Li—S cells have performed so poorly.
Despite the various approaches proposed for the fabrication of high energy density Li—S cells, there remains a need for cathode materials, production processes, and cell operation methods that retard the out-diffusion of S or lithium polysulfide from the cathode compartments into other components in these cells, improve the utilization of electro-active cathode materials (S utilization efficiency), and provide rechargeable Li—S cells with high capacities over a large number of cycles.
Most significantly, lithium metal (including pure lithium, lithium alloys of high lithium content with other metal elements, or lithium-containing compounds with a high lithium content; e.g. >80% or preferably >90% by weight Li) still provides the highest anode specific capacity as compared to essentially all other anode active materials (except pure silicon, but silicon has pulverization issues). Lithium metal would be an ideal anode material in a lithium-sulfur secondary battery if dendrite related issues could be addressed.
Sodium metal (Na), potassium metal (K), magnesium metal (Mg), and aluminum metal (Al) have similar chemical characteristics to Li, and the sulfur cathode in room temperature sodium-sulfur cells (RT Na—S batteries), potassium-sulfur cells (K—S), magnesium-sulfur cell, and aluminum-sulfur cell face the same issues observed in Li—S batteries, such as: (i) low active material utilization rate, (ii) poor cycle life, and (iii) low Coulombic efficiency. Again, these drawbacks arise mainly from insulating nature of S, dissolution of S and metal polysulfide intermediates in liquid electrolytes (and related Shuttle effect), and large volume changes during charge/discharge.
Hence, an object of the present invention is to provide a rechargeable metal-sulfur battery (e.g. Li—S, Na—S, K—S, Mg—S, or Al—S battery) that exhibits an exceptionally high specific energy density or high volumemetric energy density. One particular technical goal of the present invention is to provide a metal-sulfur or metal ion-sulfur cell with a cell specific energy greater than 400 Wh/Kg, preferably greater than 500 Wh/Kg, and more preferably greater than 600 Wh/Kg (all based on the total cell weight).
Another object of the present invention is to provide a metal-sulfur cell that exhibits a high cathode specific capacity (higher than 1,200 mAh/g based on the sulfur weight, or higher than 1,000 mAh/g based on the cathode composite weight, including sulfur, conducting additive or substrate, and binder weights combined, but excluding the weight of cathode current collector). The specific capacity is preferably higher than 1,400 mAh/g based on the sulfur weight alone or higher than 1,200 mAh/g based on the cathode composite weight. This must be accompanied by a high specific energy, good resistance to dendrite formation, and a long and stable cycle life.
It may be noted that in most of the open literature reports (scientific papers) and patent documents, scientists or inventors choose to express the cathode specific capacity based on the sulfur or lithium polysulfide weight alone (not the total cathode composite weight), but unfortunately a large proportion of non-active materials (those not capable of storing lithium, such as conductive additive and binder) is typically used in their Li—S or Na—S cells. For practical use purposes, it is more meaningful to use the cathode composite weight-based capacity value.
A specific object of the present invention is to provide a rechargeable metal-sulfur cell based on rational materials and battery designs that overcome or significantly reduce the following issues commonly associated with conventional metal-S cells: (a) dendrite formation (internal shorting); (b) extremely low electric and ionic conductivities of sulfur, requiring large proportion (typically 30-55%) of non-active conductive fillers and having significant proportion of non-accessible or non-reachable sulfur or metal polysulfides); (c) dissolution of S and metal polysulfide in electrolyte and migration of polysulfides from the cathode to the anode (which irreversibly react with Li, Na, K, Mg, or Al at the anode), resulting in active material loss and capacity decay (the shuttle effect); and (d) short cycle life.
Additionally, the production of graphene sheets typically involves the use of undesirable chemicals, such as sulfuric acid and potassium permanganate, and the efflux of regulated gases, such as SO2 and NO2. FIG. 1 illustrates a commonly used process for graphene production. Thus, an urgent need exists to have a new class of carbon nano materials that are comparable or superior to graphene in terms of properties, but can be produced more cost-effectively, faster, more scalable, and in a more environmentally benign manner. The production process for such a new carbon nano material requires a reduced amount of undesirable chemical (or elimination of these chemicals all together), shortened process time, less energy consumption, reduced or eliminated effluents of undesirable chemical species into the drainage (e.g., sulfuric acid) or into the air (e.g., SO2 and NO2). Furthermore, one should be able to readily make this new nano material into a foam structure that is essentially a 3D network of electron-conducting pathways and, hence, thermally and electrically conductive.
Generally speaking, a foam or foamed material is composed of pores (or cells) and pore walls (a solid material). The pores can be interconnected to form an open-cell foam. As an example, graphene foam is composed of pores and pore walls that contain a graphene material. We envision that graphene, when made into a foam structure, may be a good protective material for sulfur. However, most of the methods of producing graphene foams are all tedious, energy-intensive, and slow. Every prior art method or process for producing graphene and graphene foams has major deficiencies. Thus, it is an object of the present invention to provide a new class of foam material that is thermally and electrically conducting, mechanically robust, and chemically compatible with sulfur or polysulfide. Another object is to provide a cost-effective method of producing this class of foam to protect sulfur or polysulfide.
Humic acid (HA) is an organic matter commonly found in soil and coal products. HA can be extracted from the soil using a base (e.g. KOH). HA can also be extracted, with a high yield, from a type of coal called leonardite, which is a highly oxidized version of lignite coal. HA extracted from leonardite contains a number of oxygenated groups (e.g. carboxyl groups) located around the edges of the graphene-like molecular center (SP2 core of hexagonal carbon structure). This material is slightly similar to graphene oxide (GO) which is produced by strong acid oxidation of natural graphite. HA has a typical oxygen content of 5% to 42% by weight (other major elements being carbon and hydrogen). HA, after chemical or thermal reduction, has an oxygen content of 0.01% to 5% by weight. For claim definition purposes in the instant application, humic acid (HA) refers to the entire oxygen content range, from 0.01% to 42% by weight. The reduced humic acid (RHA) is a special type of HA that has an oxygen content of 0.01% to 5% by weight.
The present invention is directed at a new class of graphene-like 2D materials (i.e. humic acid) that surprisingly can be converted into a foamed structure of high structural integrity. Thus, another object is to provide a cost-effective process for producing such a nano carbon foam (specifically, humic acid-derived foam) in large quantities. This process does not involve the use of an environmentally unfriendly chemical. This method enables the flexible design and control of the porosity level and pore sizes.
It is another object of the present invention to provide a humic acid-derived foam that exhibits a thermal conductivity, electrical conductivity, elastic modulus, and/or strength comparable to or greater than those of the conventional graphite foams, carbon foams, or graphene foams. Yet another object of the present invention is to provide a humic acid-derived foam that preferably has a meso-scaled pore size range (2-50 nm). The HA-derived foams must also be capable of retaining, confining, or protecting sulfur or sulfide to solve metal-sulfur cell issues.